1. Field of the Invention
This invention relates to an implant fabricated from bone. More particularly, the present invention relates to a stable, yet flexible, prosthetic implant having little if any capacity for promoting new bone growth that could result in the unwanted fusion of bone structures at the implantation site.
2. Description of the Related Art
The vertebral column (spine) is a biomechanical structure composed of a series of joints known as motion segment units. Each motion segment unit includes two adjacent vertebrae and their facet capsules, the intervertebral disc, and connecting ligament tissue. The spine is divided into four major regions which include the cervical, thoracic, lumbar, and sacral regions and functions to protect the spinal cord and nerve roots and provide support to the body.
The intervertebral disc is a fibro-cartilaginous structure located between the two end plates of adjacent vertebrae and is composed of the nucleus pulposus (a gel-like ball located at the center of the disc) and the annulus fibrous (collagen fibers surrounding the nucleus pulposus). The intervertebral disc separates, cushions, and stabilizes the spine providing the spine with resiliency and the ability to withstand compression, rotation, and bending strain. Various types of spinal disorders are known and include kyphosis (backward curvature of the spine), spondylolysis, spondylolisthesis (forward displacement of lumbar vertebrae), scoliosis (abnormal curvature of the spine), and disorders involving ruptured, slipped, damaged and diseased discs, damaged vertebrae, and the like. Patients suffering from the aforementioned disorders typically experience severe pain, numbness, decreased mobility, muscle weakness, and nerve damage and may be treated by surgical removal of the discs. Removal of the discs may then require fusion of the bony elements of the spine.
Spinal fusion can be accomplished within the disc space, anteriorly between adjacent vertebral bodies and/or posteriorly between consecutive processes, e.g., transverse processes, laminae or other posterior elements of the vertebrae. One frequently used spinal fusion technique involves removal of the intervertebral disc and insertion of an anterior supporting structure, e.g., bone grafts, bone substitutes, plugs, bone dowels, cages, and the like, into the intervertebral disc space to prevent collapse of the disc space and promote fusion of the adjacent vertebrae. Unfortunately, this procedure virtually precludes any degree of spinal flexibility.
Other diseases of the spinal joints also require new methods of treatment. For example, the present treatment of lumbar disc herniation provides a compromised functional recovery at best. The disc that has come out of its place has no physiological function and at this time no method is known that will bring back the functional lumbar disc. Fusion is unable to preserve the natural structure and function of the spine. Therefore, a disc prosthesis is needed which can be applied to the disc space and provide the cushioning effect for the disc that is expected of the normal disc.
Several unacceptable attempts have been made to solve this problem such as applying spring loaded disc prosthesis or saline injectable discs with suction cups on the surfaces. The spring loaded disc is so bulky it cannot be inserted into the disc space through the limited opening that is available for this kind of surgery. The saline injectable disc is unstable and the suction cups do not hold it onto the vertebral bodies which have irregular spiky surfaces. While various artificial polymers have been proposed for use as a disc segment, these tend not to have the right biological properties to provide biological attachment to adjacent vertebras or to encourage growth of a supplementary fibrous tissue matrix.
U.S. Pat. No. 4,309,777 describes an artificial disc having a plurality of springs positioned between lower and upper disc portions. In addition, a plurality of spikes extends from the upper and lower portions of the disc to engage the vertebrae. U.S. Pat. No. 4,759,769 describes an artificial spinal disc comprising upper and lower portions connected by both hinge and spring devices. The discs of U.S. Pat. Nos. 4,309,777 and 4,759,769, while stable, lack a physiological structure which probably accounts for their apparent lack of acceptance by the medical community.
U.S. Pat. No. 4,834,757 describes vertebral implant plugs or blocks that are useful in fusing together adjoining vertebral bodies. Once fusion has occurred, articulation of the vertebral joint is no longer possible.
U.S. Pat. No. 4,863,476 describes a spinal implant intended for insertion in an intervertebral space. However, the implant lacks the ability to maintain full contact with the vertebral surfaces.
U.S. Pat. No. 4,863,477 describes a synthetic intervertebral disc prosthesis composed of a rubber-type material having a hollow interior. The interior may be filled with fluid imparting a certain degree of resiliency to the prosthesis. One disadvantage of the prosthesis, however, is that in the anatomical context of the intervertebral space, its design provides very little stability.
A need continues to exist for a truly stable yet fully flexible prosthetic implant that better preserves the natural structure and function of skeletal joints such as the vertebral joints.
It is an object of the invention to provide an implant fabricated from bone, one which is both flexible and resilient and capable of bearing mechanical loads while possessing little if any capacity for promoting new bone growth that could result in the unwanted fusion of bone structures with which the implant comes into contact following its implantation.
It is a particular object of the invention to provide an implant useful in the repair or replacement of skeletal joints.
It is a further object of the invention to provide a replacement intervertebral disc which does not cause the fusion of adjacent vertebrae following its implantation.